High-speed safety circuit for a hydraulic press

ABSTRACT

A high speed safety circuit arrangement is provided for a hydraulic press which is driven by a hydraulic cylinder assembly constituted by a principal cylinder unit and a subsidiary cylinder unit. The circuit arrangement comprises a servo valve disposed in one of at least a pair of conduit lines for supplying a pressure fluid from a source thereof into the principal cylinder unit and the subsidiary cylinder unit, a first logic valve disposed in the other of the conduit lines and switched on and off and by the servo valve and a first electromagnetic valve for opening and closing that other conduit line, a second and a third logic valve of which each is switched on and off, and at least one is selectively actuatable, by a second electromagnetic valve to interconnect an upper and a lower chamber of the principal cylinder unit in a hydraulic circuit.

TECHNICAL FIELD

The present invention relates to a high speed safety circuit arrangementfor a hydraulic press.

BACKGROUND ART

There have hitherto been known certain hydraulic circuit arrangementsfor a hydraulic press in which a slide is vertically driven up and downby a hydraulic cylinder assembly, as disclosed, for example, in JapaneseExamined Utility Model Publication No. Hei 2-18801, Japanese UnexaminedUtility Model Publication No. Hei 6-39285 and Japanese Unexamined PatentPublication No. Hei 6-155089.

In particular, Japanese Examined Utility Model Publication No. Hei2-18801 has described a fluid pressure control circuit arrangementhaving a construction in which a circuit for supplying a pressure fluidinto a hydraulic cylinder assembly is provided with a directionalcontrol valve and a pilot check valve such that by switching thedirectional control valve so as to allow the fluid pressure to beapplied via the check valve to the hydraulic cylinder assembly, thelatter may act to drive any particular load operatively associatedtherewith.

Also, Japanese Unexamined Utility Model Publication No. Hei 6-39285 hasspecifically disclosed a hydraulic circuit arrangement for a press inwhich the cylinder assembly is constructed to include a pressurecylinder unit having a larger pressure receiving area and a subsidiarycylinder unit having a smaller pressure receiving area but operable at ahigher speed, the two cylinder units being arranged on a common centerline. And, there a construction is employed in which the respectivepistons of the two cylinder units are coupled together by a piston rodin the two rod cylinder assembly with the piston rod in the high speedcylinder unit projecting upwards thereof.

And, the system is so configured that after the high speed cylinder unitis supplied with the pressure fluid so as to allow its piston to bedriven at a higher speed, the pressure cylinder unit may be suppliedwith the pressure fluid to allow a larger pressing force to be derivedso as to be capable of meeting with a greater load.

Further, a specific hydraulic system as disclosed in Japanese UnexaminedPatent Publication No. Hei 6-155089 has also adopted a cylinder assemblyconstituted of a high speed cylinder unit and a pressure cylinder unit.In this specific arrangement there is provided at the side of the pistonin the pressure cylinder unit a sequence valve that is adapted to beopened and closed by using a pilot pressure.

And, the system specifically disclosed there is so configured that bypermitting the sequence valve to be switched on and off, a high speedoperation may be switched to a pressing operation, thus being capable ofmeeting with a greater load at a higher speed without requiring aspecial piping unit or valves externally fitted.

With a fluid pressure control circuit arrangement as disclosed inJapanese Examined Utility Model Publication No. Hei 2-18801 it is noted,however, that where a dust is introduced onto a spool in the directionalcontrol valve so as to disturb its downward displacement the pressurefluid from the pressure cylinder may no longer be constricted, thusbringing about the inconvenience that there can arise the danger of anincreased load.

With a hydraulic circuit arrangement as disclosed in Japanese UnexaminedUtility Model Publication No. Hei 6-39285, it is also noted that a biteinto a die may arise during a pressing operation while a large force ofremoval may not be obtained. There then arises an inconvenience that apressed sheet may not be removed from its die bite.

It further is noted that with a high speed for a greater load cylinderassembly as disclosed in Japanese Unexamined Patent Publication No. Hei6-155089 in which the sequence valve is provided internally of thepiston in the pressure cylinder unit, not only is its maintainabilitypoor but also there may be brought about a danger arising from the factthat the piston rod in the high speed cylinder unit is projectingupwards thereof.

With the above described problems taken into account it is an object ofthe present invention to provide a high speed safety circuit arrangementfor a hydraulic press that is capable of permitting a slide to be drivenand operated at an increased speed and yet with due safety.

SUMMARY OF THE INVENTION

In order to achieve the above mentioned object, there is provided a highspeed safety circuit arrangement for a hydraulic press in which a slideis vertically driven by a hydraulic cylinder assembly constituted of aprincipal cylinder unit and a subsidiary cylinder unit which arearranged vertically down and up with a common center line therefor, thesaid principal cylinder unit having a pressure receiving area which isgreater than that of the said subsidiary cylinder unit, and a piston inthe said principal cylinder unit and a piston in the said subsidiarycylinder unit being connected together by a piston rod in the saidsubsidiary cylinder unit that is smaller in diameter than a piston rodin the said principal cylinder unit, which circuit arrangementcomprises: a servo valve means disposed in one of at lest a pair ofconduit lines for supplying a pressure fluid from a source thereof intosaid principal cylinder unit and said subsidiary cylinder unit,respectively, and operable to switch the direction in which the saidpressure fluid is supplied; a first electromagnetic valve means; a firstlogic valve means disposed in the other of the said conduit lines andadapted to be switched on and off in response to an operation of thesaid servo valve means and the said first electromagnetic valve means; asecond electro-magnetic valve means; and a second and a third logicvalve means of which each is adapted to be switched on and off inresponse to an operation of the said second electromagnetic valve meansand at least one is selectively actuatable thereby to interconnect anupper and a lower chamber of the said principal cylinder unit in ahydraulic circuit.

And, in the construction described above, it is desirable that there befurther provided a third electromagnetic valve; and a first pilot checkvalve means disposed midway in a conduit line interconnecting the saidpressure fluid source and the said servo valve means and adapted to beswitched on and off in response to an operation of the said thirdelectromagnetic valve means for opening and closing the last mentionedconduit line.

It is also desirable that there be further provided a fourthelectromagnetic valve means for connecting the upper chamber in the saidprincipal cylinder unit and an upper chamber in the said subsidiarycylinder unit with each other; a fifth electromagnetic valve means; anda second check valve means adapted to be switched on and off in responseto the said fifth electromagnetic means for interconnecting the upperchamber of the said subsidiary cylinder unit and a fluid reservoir viathe said second pilot check valve means.

According to a construction as described above, it may be seen that bypermitting the pressure fluid to be supplied into the upper chamber andthe lower chamber of the principal cylinder unit via the one conduitline by means of the servo valve means, the slide with the principalcylinder unit can be moved down at an increased speed owing to adifference in the pressure receiving area between the two chambers.

It may also be seen that during a given pressing operation, an increasedpressing force can be obtained with the pressure fluid supplied into theupper chamber of the principal cylinder unit that has a larger pressurereceiving area. Also, since while the slide is moving upwards a greaterrising force therefor is attained with the pressure fluid supplied intoboth the respective lower chambers of the principal cylinder unit andthe subsidiary cylinder unit, it may be seen that the upper pressing diecan be detached readily from a pressed workpiece if the former werebitten into the latter.

Further, since a given electromagnetic valve means is provided for eachconduit line independently of others, it may be seen that if any onecomes into a failure during a given pressing operation, the otherelectromagnetic valves coupled with the servo valve means caneffectively act to stop the pressing operation.

In a construction as described above, it should also be noted that thesaid logic valve means for interconnecting the upper and lower chambersof the principal cylinder unit in the hydraulic circuit can beconstituted by the said second and third logic valve means togetherwhich are connected in series with each other and substantially of anidentical size, and that the said second and third logic valve means canbe switched on and off alternately in response to an operation of thesaid second electromagnetic means for effecting a pressure compensationin the said hydraulic circuit.

According to the construction described in the preceding paragraph, itmay be seen that with the use of the said logic valve means which areconnected in series with each other and of which one is designed for apressure compensation as noted, it is also possible to prevent thepressure in a said cylinder unit from being suddenly elevated due to avolume change therein when the logic valves are operated.

Also, in a construction as described above, it may be noted that thesaid logic valve means for interconnecting the upper and lower chambersof the said principal cylinder unit can be constituted by said thirdlogic valve means, and that there can further be a shuttle valve meansfor applying therethrough the pressure fluid as a back pressure mediumto the said third logic valve means at a high pressure side thereof foreffecting a pressure compensation in the said hydraulic circuit.

According to the construction described in the preceding paragraph, itmay be seen that since the logic valve means for interconnecting theupper and lower chambers of the principal cylinder unit is utilized toreceive a back pressure for effecting a pressure compensation as noted,any separate logic valve means that is specifically for such a pressurecompensation can be effectively dispensed with.

BRIEF EXPLANATION OF THE DRAWINGS

The present invention will better be understood from the followingdetailed description and the drawings attached hereto showing certainillustrative embodiments of the present invention. In this connection,it should be noted that such embodiments as illustrated in theaccompanying drawings are intended in no way to limit the presentinvention but to facilitate an explanation and understanding thereof.

In the accompanying drawings:

FIG. 1 is a circuit diagram that illustrates a first embodiment of thehigh speed safety circuit arrangement for a hydraulic press according tothe present invention;

FIGS. 2A and 2B together represent an explanatory view that illustratesan operation in which the slide is allowed to descend at an increasedspeed according to the above noted first embodiment of the presentinvention;

FIGS. 3A and 3B together represent an explanatory view that illustratesan operation in which the slide is allowed to descend at a reduced speedaccording to the above noted first embodiment of the present invention;

FIGS. 4A and 4B together represent an explanatory view that illustratesan operation in which the slide is allowed to ascend at a reduced speedaccording to the above noted first embodiment of the present invention;

FIGS. 5A and 5B together represent an explanatory view that illustratesan operation in which the slide is allowed to ascend at an increasedspeed according to the above noted first embodiment of the presentinvention;

FIG. 6 is a circuit diagram that illustrates a second embodiment of thehigh speed safety circuit arrangement for a hydraulic press according tothe present invention;

FIGS. 7A and 7B together represent an explanatory view that illustratesan operation in which the slide is allowed to descend at an increasedspeed according to the above noted second embodiment of the presentinvention;

FIGS. 8A and 8B together represent an explanatory view that illustratesan operation in which the slide is allowed to descend at a reduced speedaccording to the above noted second embodiment of the present invention;

FIGS. 9A and 9B together represent an explanatory view that illustratesan operation in which the slide is allowed to ascend at a reduced speedaccording to the above noted second embodiment of the present invention;

FIGS. 10A and 10B together represent an explanatory view thatillustrates an operation in which the slide is allowed to ascend at anincreased speed according to the above noted second embodiment of thepresent invention;

FIG. 11 is a circuit diagram that illustrates a third embodiment of thehigh speed safety circuit arrangement for a hydraulic press according tothe present invention;

FIGS. 12A and 12B together represent an explanatory view thatillustrates an operation in which the slide is allowed to descend at anincreased speed according to the above noted third embodiment of thepresent invention;

FIGS. 13A and 13B together represent an explanatory view thatillustrates an operation in which the slide is allowed to descend at areduced speed according to the above noted third embodiment of thepresent invention;

FIGS. 14A and 14B together represent an explanatory view thatillustrates an operation in which the slide is allowed to ascend at areduced speed according to the above noted third embodiment of thepresent invention; and

FIGS. 15A and 15B together represent an explanatory view thatillustrates an operation in which the slide is allowed to ascend at anincreased speed according to the above noted third embodiment of thepresent invention;

BEST MODES FOR CARRYING OUT THE INVENTION

Hereinafter, suitable embodiments of the present invention with respectto a high speed safety circuit arrangement for a hydraulic press will beset forth with reference to the accompanying drawings hereof.

An explanation will now be given of the first embodiment of the presentinvention with reference to FIGS. 1 through 5B.

Referring first to FIG. 1, a hydraulic cylinder assembly 1 is shown asconstituted of a principal cylinder unit 3 that has a larger pressurereceiving area and a subsidiary cylinder unit 2 that has a smallerpressure receiving area.

The subsidiary cylinder unit 2 and the principal cylinder unit 3 whichconstitutes the hydraulic cylinder assembly 1 are provided vertically upand down on a common center line therefor. These cylinder units 2 and 3have pistons 2a and 3a received therein, respectively.

The piston 2a received within the subsidiary cylinder unit 2 has adownwards projecting piston rod 2b attached to its lower surface and thepiston rod 2b has its lower end attached to the upper surface of thepiston 3a received within the principal cylinder unit 3 whereas thepiston 3a within the principal cylinder unit 3 has its lower surface towhich is attached a downwards projecting piston rod 3b that is greaterin diameter than the above mentioned piston rod 2b. The piston rod 3bprojecting downwards has its lower end penetrating out of the cylinderof the principal cylinder unit 3 through its lower end plate 3c andhaving attached thereto a press slide 9.

It can also be seen that a pressure fluid discharged from a fluidpressure source 4 which can be a variable flow rate hydraulic pump isdelivered into a servo valve assembly 8 via a conduit line 7 in which apilot check valve 6 is provided.

The servo valve assembly 8 comprises a main valve 8a, a pilot switchingvalve 8b that consists of an electromagnetic valve for switching under apilot pressure, and an ON/OFF valve 8d that consists of anelectromagnetic valve which is provided midway of a pilot circuit 8c.

And, a two line conduit system 10 for connecting the servo valveassembly 8 and the hydraulic cylinder assembly 1 with each othercomprises a conduit line 10₁ that is connected to an upper chamber 3₁ ofthe principal cylinder unit 3 and the logic valve 5 via a pair of logicvalves 15 and 14 which are alternately opened and closed with anelectromagnetic valve 13, and a conduit line 10₂ that is connected to alower chamber 2₂ of the subsidiary cylinder unit 2 and that isconnectable to a lower chamber 3₂ of the principal cylinder unit 3 via alogic valve 17 which is opened and closed with an electromagnetic valve16. It will also be noted that an upper chamber 2₁ of the subsidiarycylinder unit 2 is opened to the atmosphere.

On the other hand, the upper chamber 3₁ and the lower chamber 3₂ of theprincipal cylinder unit 3 are provided with sensing means 19 and 20,respectively, each of which consists of a pressure sensor and which areadapted to sense pressures within the chambers 3₁ , and 3₂,respectively, thereby to detect a pressing force P applied by the slide9. In addition, there is provided in the vicinity of the slide 9 afurther sensing means for detecting a position of the slide 9. Signalsthat are representative of the pressure and the position of the slide 9are furnished to enter into a controller 22.

An explanation will next be given with respect to an operation of thehigh speed safety circuit arrangement for a press, that is constructedas noted above. In this regard, it should be noted that the word "ON"represents "open" or "opened" and the word "OFF" represents "close" or"closed".

In a case where the slide 9 is being allowed to descend from an upperdead point to initiate a given pressing operation, the pilot switchingvalve 8b and the ON/OFF valve 8d of the servo valve assembly 8 will beturned ON to switch the main valve 8a to its descending position 8₁ fromits neutral position 8₃. At the same time, the pilot check valve 6 willbe turned ON by the electromagnetic valve 5, the logic, valve 14 will beturned OFF and the logic valve 15 will be turned ON by theelectromagnetic valve 13, and the logic valve 17 will be turned OFF bythe electromagnetic valve 16.

This will, as shown in FIG. 2A, cause the pressure fluid discharged fromthe fluid pressure source 4 to flow via the logic valves 15 and 14through the conduit line 10₁ into the upper chamber 3₁ of the principalcylinder 3. Also, a fluid communication has been established between theupper chamber 3₁ and the lower chamber 3₂ via the logic valves 14 and15. Accordingly the slide 9 will, as shown by the thick line in FIG. 2B,be lowered at a high speed owing to a difference in the pressurereceiving area between the upper chamber 3₁ and the lower chamber 3₂ ofthe principal cylinder unit 3. The pressure fluid in the lower chamber2₂ of the subsidiary cylinder unit 2 will then be drained through theconduit line 10₂ into a reservoir 18 via the servo valve assembly 8.

Next, in a case where the slide 9 has been moved down to a predeterminedposition and a pressing force is then required to form a workpiece, itshould be noted that with the main valve 8a in the servo valve assembly8 held at its descending position, the logic valve 14 will be turned ONand the logic valve 15 will be turned OFF by the electromagnetic valve13 while the logic valve 17 will be turned ON by the electromagneticvalve 16.

This will, as shown in FIG. 3A, cause the pressure fluid discharged fromthe fluid pressure source 4 to be supplied only into the upper chamber3₁ of the principal cylinder unit 3, the pressure fluid in the lowerchamber 3₂ of the principal cylinder unit 3 to flow out thereof via thelogic valve 17 into the conduit line 10₂ and then along with thepressure fluid in the lower chamber 2₂ of the subsidiary cylinder 10₂ tobe drained into the reservoir 18. As a result, the piston 3a will bepushed downwards under a pressure of the fluid in the upper chamber 3₁of the principal cylinder unit 3 to cause the slide 9 to descend at areduced speed as shown by the thick line in FIG. 3B. Then an increasedpressing force will be produced to allow a workpiece to be formedbetween an upper die and a lower die (none of them shown)satisfactorily.

Also, in a case where the workpiece is required to be held in apressurized state during a given forming operation, in the servo valveassembly 8 the main valve 8a will be switched to its neutral position 8₃by the pilot switching valve 8b. Then, since the slide 9 is stationaryat the position taken at the time, the workpiece can be held in apressed state.

On the other hand, in a case where upon forming a workpiece the slide 9is being allowed to ascend from a lower dead point, in the servo valveassembly 8 the main valve 8a will be switched to its ascending positionby the pilot switching valve 8b, and the logic valve 14 will be turnedON and the logic valve 15 will be turned OFF by the electromagneticvalve 16.

This will, as shown in FIG. 4A, cause the pressure fluid discharged fromthe fluid pressure source 4 to be supplied into the lower chamber 2₂ ofthe subsidiary cylinder unit 2 through the conduit line 10₂ and into thelower chamber 3₂ of the principal cylinder unit 3 via the logic valve 17while permitting the fluid in the upper chamber 3₁ of the principalcylinder 3 to be drained into the reservoir 18 through the conduit line10₁.

This will in turn cause the slide 9 to rise at a reduced speed as shownby the thick line in FIG. 4B. Then, since a lifting force of theprincipal cylinder unit 3 is added to the lifting force of thesubsidiary cylinder unit 2, it may be seen that even where there is abite of the upper die into the workpiece during a given formingoperation, the bitted upper die can be intensively detached from theworkpiece.

Thereafter, in the state in which the main valve 8a in the servo valveassembly 8 is held at its ascending position 8₂, the logic valve 14 willbe turned OFF and the logic valve 15 will be turned ON by theelectromagnetic valve 13, and the logic valve 17 will be turned OFF bythe electromagnetic valve 16. This will, as shown in FIG. 5A, cause thepressure of the fluid discharged from the fluid pressure source 4 to beapplied via the conduit line 10₂ to the lower chamber 2₂ of thesubsidiary cylinder unit 2, the pressure fluid in the upper chamber 3₁of the principal cylinder 3 to flow via the logic valves 14 and 15 intoits lower chamber 3₂, an excessive amount of the fluid produced from theupper chamber 3₁ due to a difference in the pressure receiving areabetween the chambers 3₁ and 3₂ to be drained through the conduit line10₁ into the reservoir 18. As a result, the slide 9 will be allowed toascend rapidly up to its upper dead point as shown by the thick line inFIG. 5B.

The foregoing is a description of a series of normal operations as faras the first embodiment of the present invention is concerned in whichthere are provided a meter-in side circuit and a meter-out side circuitindependently of each other and in which the meter-in side circuit isprovided with the pilot check valve 6 and the servo valve assembly 8whereas the meter-out circuit is provided with the logic valves 15 and17 operating as a counter-balance valve and the servo valve assembly 8.

There is also provided the ON/OFF valve 8d that consists of anelectromagnetic valve between the main valve 8a and the pilot switchingvalve 8b in the servo valve assembly 8.

This will, if either the electromagnetic valve 13 or 16 comes into afailure, allow the slide 9 to be stopped safely by the otherelectromagnetic valve 13 or 16 coupled with the servo valve assembly 8and at the same time will, when the hydraulic pressure fluid happens toact abnormally, allow the main valve 8a in the servo valve assembly 8 tobe returned to its neutral position without fail by turning the ON/OFFvalve 8d OFF. Hence there will develop a doubled safety function thatcan operate. Als, where the main valve 8a comes to fail, it may be seenthat the slide 9 can be stopped by turning the electromagnetic valves 5,13 and 16 OFF.

It should be noted at this point that while not shown in the above notedfirst embodiment of the present invention, the logic valves 14, 15 and17 are actually provided in a manifold block that is directly attachedto the body of the cylinder assembly 1 so that it may be madeunnecessary to provide a special piping unit externally installed toreduce the pressure loss and that it may also be easy to make amaintenance for these logic valves.

It should also be noted that the logic valves 14 and 15 are connected inseries and are of an identical size and one of them 14 is used to effecta pressure compensation in the hydraulic circuit.

More specifically, assuming that the volume which varies as the elementsof the logic valve 14 are displaced from the state in which therelationship of PVn=const. applies is expressed as Δ V and the pressureP and the volume V before and after the displacement of the elements areexpressed as P1, V1; P2, V2, respectively,

    P1V1.sup.n =P2V2.sup.n, V2=V1-Δ V

Therefore,

    P2=P1V1.sup.n /(V1-Δ V).sup.n (>P1)

As may be noted, this can prevent a shock from being generated due to asudden change in the pressure because the logic valve 14 that is turnedON and OFF alternately with the logic valve 15 can be pressurecompensated for a change in the volume produced when the logic valve 15is turned ON and OFF.

FIGS. 6 through 10B show a second embodiment of the present invention.

It should be noted here that while the second embodiment is described,the same components as used in the first embodiment will have the samereference characters and the explanation of their details will beomitted.

While in the first embodiment the upper chamber 2₁ of the subsidiarycylinder unit 2 is opened to the atmosphere, in the second embodimentthe upper chamber 2₁ and the lower chamber 2₂ of the subsidiary cylinderunit 2 are interconnected by a fluid conduit 26 with an electromagneticvalve 25 provided midway thereof. And, the conduit line 26 connected tothe upper chamber 2₁ is further branched with a branching conduit line26a being connected to the reservoir 18 via a pilot check valve 28 thatis turned ON and OFF by an electromagnetic valve 27.

Next, an explanation will be given with respect to an operation of theabove described second embodiment with to FIGS. 7A through 10B.

In a case where the slide 9 is being allowed to descend from its upperdead point to initiate a given pressing operation, the pilot switchingvalve 8b and the ON/OFF valve 8d in the servo valve assembly 8 will beturned ON to switch the main valve 8a from its neutral position 8₃ toits descending position 8₁. At the same time, the logic valve 14 will beturned OFF and the logic valve 15 will be turned ON by theelectromagnetic valve 13, the logic valve 17 will be turned OFF by theelectromagnetic valve 16, the electromagnetic valve 25 will be turnedOFF, and the pilot check valve 28 will be turned ON by theelectromagnetic valve 27.

This will, a shown in FIG. 7A, cause the pressure fluid discharged fromthe fluid pressure source 4 to flow through the conduit line 10₁ intothe upper chamber 3₁ of the principal cylinder unit 3 via the logicvalves 15 and 14. Also, since the upper chamber 3₁ and the lower chamber3₂ are interconnected via the logic valves 14 and 15, the slide 9 willbe allowed to descend at an increased speed as shown by the thick linein FIG. 7B owing to a difference in the pressure receiving area betweenthe upper chamber 3₁ and the lower chamber 3₂ of the principal cylinderunit 3. Then, the fluid in the lower chamber 2₂ of the subsidiarycylinder unit 2 will be drained through the conduit line 10₂ into thereservoir 18 via the servo valve assembly 8 and also the fluid in thereservoir 18 will be sucked into the upper chamber 2₁ of the subsidiarycylinder unit 2 via the pilot check valve 28.

Next, in a case where the slide 9 has been lowered to a predeterminedposition and a given pressing force is required to form a workpiece, itmay be seen that with the main valve 8a held at its descending position8₁ in the servo valve assembly 8, the logic valve 14 will be turned ONand the logic valve 15 will be turned OFF by the electromagnetic valve13, the logic valve 17 will be turned ON by the electromagnetic valve16, the electromagnetic valve 25 will be turned ON, and the pilot checkvalve 28 will be turned OFF by the electromagnetic valve 27.

This will, as shown in FIG. 8A, cause the pressure fluid discharged fromthe fluid pressure source 4 to be supplied via the logic valves 15 and14 into the upper chamber 3₁ of the principal cylinder 3 and to besupplied via the electromagnetic valve 25 into the upper chamber 2₁ ofthe subsidiary cylinder unit 2, and the fluid in the lower chamber 3₂ ofthe principal cylinder 3 to flow out thereof into the conduit line 10₂via the logic valve 17 and along with the fluid in the lower chamber 2₂of the subsidiary cylinder 2 to be drained into the reservoir 18. As aresult, the piston 3a will be pushed downwards under a pressure of thefluid in the upper chamber 3₁ of the principal cylinder unit 3 and thefluid in the upper chamber 2₁ of the subsidiary cylinder unit 2 to causethe slide 9 to descend at a reduced speed as shown by the thick line inFIG. 8B. Then, an increased pressing force will be generated to allow aworkpiece to be formed between the upper die and the lower die (none ofthem shown).

Also, in a case where the workpiece is required to be held in apressurized state during a given forming operation, the main valve 8a inthe servo valve assembly 8 will be switched to its neutral position 8₃by the pilot switching valve 8b to cause the slide 9 to be stopped atthe position taken at the time to hold the workpiece in the state inwhich it is pressed.

On the other hand, in a case where upon forming the workpiece the slide9 is being allowed to ascend from its lower dead point, the main valve8a in the servo valve assembly 8 will be switched to its ascendingposition 8₂ by the pilot switching valve 8b, the electromagnetic valve25 will be turned ON and with the pilot check valve 28 held OFF thelogic valve 17 will be turned ON by the electromagnetic valve 16 and thelogic valve 14 will be turned ON and the logic valve 15 will be turnedOFF by the electromagnetic valve 13.

This will, as shown in FIG. 9A, cause the pressure fluid discharged fromthe fluid pressure source 4 to be supplied through the conduit line 10₂into the lower chamber 2₂ of the subsidiary cylinder unit 2 and to besupplied via the logic valve 17 into the lower chamber 3₂ of theprincipal cylinder unit 3 and allow the fluid in the upper chamber 2₁and the fluid in the upper chamber 3₁ of the principal chamber 3 to bedrained through the conduit line 10₁ into the reservoir 18.

This will in turn cause the slide 9 to ascend at a reduced speed asshown by the thick line in FIG. 9B. Then, since a lifting force of theprincipal cylinder unit 3 is added to the lifting force of thesubsidiary cylinder unit 2, it may be seen that even where there is abite of the upper die into the workpiece during a given formingoperation, the bitted upper die can be intensively detached from theworkpiece.

Thereafter, in the state in which the main valve 8a in the servo valveassembly 8 is held at its ascending position 8₂, the logic valve 14 willbe turned OFF and the logic valve 15 will be turned ON by theelectromagnetic valve 13, the electromagnetic valve 25 will be turnedOFF, the pilot check valve 28 will be turned ON, and the logic valve 17will be turned OFF by the electromagnetic valve 16. This will, as shownin FIG. 10A, cause the pressure of the fluid discharged from the fluidpressure source 4 to be applied via the conduit line 10₂ to the lowerchamber 2₂ of the subsidiary cylinder unit 2, the pressure fluid in theupper chamber 3₁ of the principal cylinder 3 to flow via the logicvalves 14 and 15 into its lower chamber 3₂, an excessive amount of thefluid produced from the upper chamber 3₁ due to a difference in thepressure receiving area between the chambers 3₁ and 3₂ to be drainedthrough the conduit line 10₁ into the reservoir 18 and the fluid in theupper chamber 2₁ of the subsidiary cylinder 2 to be drained via thepilot check valve 28 into the reservoir 18. As a result, the slide 9will be allowed to ascend rapidly up to its upper dead point as shown bythe thick line in FIG. 10B.

It should be noted at this point that while in both the first and secondembodiments of the present invention, by turning ON and OFF the logicvalves 14 and 15 alternately which are of an identical size and areconnected in series with each other a change in pressure generated whenone of them 15 is turned ON and OFF is compensated for with the other ofthem 14, it is also possible to effect a pressure compensation for thelogic valve 15 without using the logic valve 14 in such a circuit as athird embodiment of the present invention that will be described below.

FIGS. 11 through 15 show the third embodiment of the present invention.This embodiment is designed to dispense with the logic valve 14 for thepurpose of such a pressure compensation and then to introduce thepressure fluid at a high pressure acting as a back pressure medium intothe logic valve 14 by a shuttle valve 30.

More specifically, the logic valve 15 that is provided in the conduitline 10₁ communicating with the upper chamber of the principal cylinderunit 3 will be turned ON and OFF while the pressure fluid flowing at anincreased pressure through the conduit lines 10₁ and 10₃ is introducedvia the above mentioned electromagnetic valve 13 as a back pressuremedium to a spring chamber 15a of the logic valve 15 by the shuttlevalve 30 that is connected to the conduit line 10₁ and to a conduit line10₃ which is designed to interconnect the logic valve 15 and the lowerchamber 3₂ of the principal cylinder unit 3.

Next, with reference to FIGS. 12A through 15B an explanation will begiven with respect to an operation of the third embodiment constructedas described above of the present invention.

In a case where the slide 9 is being allowed to descend from its upperdead point to initiate a given pressing operation, first the pilotswitching valve 8b and the ON/OFF valve 8d in the servo valve assembly 8will be turned to switch the spool 8a from its neutral position 8₃ toits descending position 8₁. At the same time, the logic valve 15 will beturned ON by the electromagnetic valve 13 and the logic valve 17 will beturned OFF by the electromagnetic valve 16.

This will, as shown in FIG. 12A, cause the pressure fluid dischargedfrom the fluid pressure source 4 to flow through the conduit line 10into the upper chamber 3₁ of the principal cylinder unit 3 via the logicvalve 15. Also, the upper chamber 3₁ and the lower chamber 3₂ areinterconnected via the logic valve 15. Accordingly the slide 9 will beallowed to descend at an increased speed as shown by the thick line inFIG. 12B owing to a difference in the pressure receiving area betweenthe upper chamber 3₁ and the lower chamber 3₂ of the principal cylinderunit 3. Then, the fluid in the lower chamber 2₂ of the subsidiarycylinder unit 2 will be drained through the conduit line 10₂ into thereservoir 18 via the servo valve assembly 8.

Next, in a case where the slide 9 has been lowered to a predeterminedposition and a given pressing force is required to form a workpiece, itmay be seen that with the main valve 8a held at its descending position8₁ in the servo valve assembly 8, the logic valve 15 will be turned OFFby the electromagnetic valve 13, and the logic valve 17 will be turnedON by the electromagnetic valve 16.

This will, as shown in FIG. 13A, cause the pressure fluid dischargedfrom the fluid pressure source 4 to be supplied via the logic valve 15only into the upper chamber 3₁ of the principal cylinder 3 and the fluidin the lower chamber 3₂ of the principal cylinder 3 to flow out thereofthrough the conduit line 10₃ into the conduit line 10₂ via the logicvalve 17 and along with the fluid in the lower chamber 2₂ of thesubsidiary cylinder 2 to be drained into the reservoir 18. As a result,the piston 3a will be pushed downwards under a pressure of the fluid inthe upper chamber 3₁ of the principal cylinder unit 3 to cause the slide9 to descend at a reduced speed as shown by the thick line in FIG. 13B.Then, an increased pressing force will be generated to allow a workpieceto be formed between the upper die and the lower die (none of themshown).

Also, in a case where the workpiece is required to be held in apressurized state during a given forming operation, the main valve 8a inthe servo valve assembly 8 will be switched to its neutral position 8₃by the pilot switching valve 8b to cause the slide 9 to be stopped atthe position taken at the time to hold the workpiece in the state inwhich it is pressed.

On the other hand, in a case where upon forming the workpiece the slide9 is being allowed to ascend from its lower dead point, the main valve8a in the servo valve assembly 8 will be switched to its ascendingposition 8₂ by the pilot switching valve 8b, the logic valve 17 will beturned ON by the electromagnetic valve 16, and the logic valve 15 willbe turned OFF by the electromagnetic valve 13.

This will, as shown in FIG. 14A, cause the pressure fluid dischargedfrom the fluid pressure source 4 to be supplied through the conduit line10₂ into the lower chamber 2₂ of the subsidiary cylinder unit 2 and tobe supplied via the logic valve 17 through the conduit line 10₃ into thelower chamber 3₂ of the principal cylinder unit 3 and allow the fluid inthe upper chamber 3₁ of the principal chamber 3 to be drained throughthe conduit line 10₁ into the reservoir 18.

This will in turn cause the slide 9 to ascend at a reduced speed asshown by the thick line in FIG. 14B. Then, since a lifting force of theprincipal cylinder unit 3 is added to the lifting force of thesubsidiary cylinder unit 2, it may be seen that even where there is abite of the upper die into the workpiece during a given formingoperation, the bitted upper die can be intensively detached from theworkpiece.

Thereafter, in the state in which the main valve 8a in the servo valveassembly 8 is held at its ascending position 8₂, the logic valve 15 willbe turned ON by the electromagnetic valve 13, and the logic valve 17will be turned OFF by the electromagnetic valve 16. This will, as shownin FIG. 14A, cause the pressure of the fluid discharged from the fluidpressure source 4 to be applied via the conduit line 10₂ to the lowerchamber 2₂ of the subsidiary cylinder unit 2, the pressure fluid in theupper chamber 3₁ of the principal cylinder 3 to flow via the logic valve15 and through the conduit line 10₃ into its lower chamber 3₂, anexcessive amount of the fluid produced from the upper chamber 3₁ due toa difference in the pressure receiving area between the chambers 3₁ and3₂ to be drained through the conduit line 10₁ into the reservoir 18. Asa result, the slide 9 will be allowed to ascend rapidly up to its upperdead point as shown by the thick line in FIG. 15B.

As described in the foregoing, it may be seen that even where the logicvalve 14 designed to effect a pressure compensation is dispensed with asin this embodiment, a function same as in the first embodiment isobtainable. At the same time it may be seen that with the pressure fluidflowing at an increased pressure through the conduit lines 10₁ and 10₃applied as a back pressure medium to the spring chamber 15a of the logicvalve 15 via the shuttle valve 30, a change in pressure generated whenthe logic valve 15 is operated can be compensated for.

As set forth in the foregoing description in detail, it may be seen thataccording to one aspect of the present invention in which there beingprovided in a conduit line for supplying the pressure fluid to thehydraulic cylinder assembly from the source thereof with a servo valveassembly provided as controllable independently and a logic valve thatis adapted to be turned ON and OFF by an electromagnetic valve, if aelectromagnetic valve in one given conduit line fails to operate, anelectromagnetic valve and a servo valve assembly in the other conduitline can be used to control the operation of the hydraulic cylinderassembly, thus permitting a press system to cease operating in safety.

Also, even where a servo valve assembly fails to operate, it can berestored to the neutral position without fail by blocking a pilotcircuit to allow a press to cease operating. Thus, since the safetyfunction is doubled to operate, the required safety is markedlyenhanced. Further, with logic valves used which are of an identical sizeand are connected in series with each other for a pressure compensation,it can be prevented for a pressure from being suddenly elevated due to achange in volume as generated within a cylinder assembly when a logicvalve is operated.

Further, if a pressure fluid at an increased pressure is applied as aback pressure medium to a logic valve, it may be seen that any otherlogic valve designed to effect a pressure compensation will be madeunnecessary and hence be economical. At the same time, since a slide isallowed to ascend with a greater lifting force that is developed by botha principal and a subsidiary cylinder unit while it is ascending, anypossible inconvenience arising from a bite of an upper die into aworkpiece which makes it difficult for the former to be detached fromthe latter can be eliminated.

And also, if a logic valve is included in a manifold block directlyattached to the body of a hydraulic cylinder assembly, it may be seenthat not only it can be made unnecessary to provide a special pipingunit externally installed to reduce any possible pressure loss and toenhance its maintainability but also the piston rod in the subsidiarycylinder unit is effectively prevented from projecting upwards and henceis safe.

While the present invention has hereinbefore been set forth with respectto certain illustrative embodiments thereof, it will readily beappreciated by a person skilled in the art to be obvious that manyalterations thereof, omissions therefrom and additions thereto can bemade without departing from the essence and the scope of the presentinvention. Accordingly, it should be understood that the presentinvention is not limited to the specific embodiments thereof set outabove, but includes all possible embodiments thereof that can be madewithin the scope with respect to the features specifically set forth inthe appended claims and encompasses all the equivalents thereof.

What is claimed is:
 1. In a hydraulic press in which a slide isvertically driven by a hydraulic cylinder assembly having:a principalcylinder unit and a subsidiary cylinder unit together constituting thehydraulic cylinder assembly and being arranged vertically down and upwith a common center line therefor, said principal cylinder unit havinga pressure receiving area which is greater than that of said subsidiarycylinder unit, and a piston in said principal cylinder unit and a pistonin said subsidiary cylinder unit being connected together by a pistonrod in said subsidiary cylinder unit that is smaller in diameter than apiston rod in said principal cylinder unit, a high speed safety circuitarrangement for said hydraulic press, which comprises: a servo valvemeans disposed in one of at least a pair of conduit lines for supplyinga pressure fluid from a source thereof into said principal cylinder unitand said subsidiary cylinder unit, respectively, and operable to switchthe direction in which said pressure fluid is supplied; a firstelectromagnetic valve means; a first logic valve means disposed in theother of said conduit lines and adapted to be switched on and off inresponse to an operation of said servo valve means and said firstelectromagnetic valve means; a second electromagnetic valve means; and asecond and a third logic valve means of which each is adapted to beswitched on and off in response to an operation of said secondelectromagnetic valve means and at least one is selectively actuatablethereby to interconnect an upper and a lower chamber of said principalcylinder unit in a hydraulic circuit.
 2. A high speed safety circuitarrangement for the hydraulic press, as set forth in claim 1, furthercomprising:a third electromagnetic valve; and a first pilot check valvemeans disposed midway in a conduit line interconnecting said pressurefluid source and said servo valve means and adapted to be switched onand off in response to an operation of said third electromagnetic valvemeans.
 3. A high speed safety circuit arrangement for the hydraulicpress, as set forth in claim 1, further comprising:a fourthelectromagnetic valve means for connecting the upper chamber in saidprincipal cylinder unit and an upper chamber in said subsidiary cylinderunit with each other; a fifth electromagnetic valve means; and a secondcheck valve means adapted to be switched on and off in response to saidfifth electromagnetic means for interconnecting the upper chamber ofsaid subsidiary cylinder unit and a fluid reservoir via said secondpilot check valve means.
 4. A high speed safety circuit arrangement forthe hydraulic press, as set forth in claim 1, in which said logic valvemeans for interconnecting the upper and lower chambers of said principalcylinder unit in the hydraulic circuit is constituted by said second andthird logic valve means together which are connected in series with eachother and substantially of an identical size, and said second and thirdlogic valve means are adapted to be switched on and off alternately inresponse to an operation of said second electromagnetic means foreffecting a pressure compensation in said hydraulic circuit.
 5. A highspeed safety circuit arrangement for the hydraulic press, as set forthin claim 1, in which said logic valve means for interconnecting theupper and lower chambers of said principal cylinder unit is constitutedby said third logic valve means, further comprising:a shuttle valvemeans for applying therethrough the pressure fluid as a back pressuremedium to said third logic valve means at a high pressure side thereoffor effecting a pressure compensation in said hydraulic circuit.